A hydraulic driving device of civil engineering and construction machinery, comprising hydraulic pumps (1, 2), a switching valve group (15a) which is connected to the pump (1) and includes a bypass stop valve (7), a switching valve group (15b) which is connected to the pump (2) and includes a preliminary directional control valve (11), a communication pipe line (13) establishing communication of the upstream-most side of the switching valve group (15a) with the supply pipe line (11a) of the preliminary directional control valve (11), a merge switching valve (14) allowing the communication pipe line (13) to communicate or shut off, an interlocked operation means switching the merge switching valve (14) to open position and the bypass stop valve (7) to closed position in conjuction with the switching operation of the preliminary directional control valve (11), and a selective switching valve (28) switching between the states where the switching operation of the merge switching valve (14) is enabled and disabled by the interlocked operation means, wherein the switching valve groups (15a, 15b), communication pipe line (13), and merge switching valve (14) are installed in a housing (15) so that a change in the maximum value of the operating speed of an actuator controlled by the preliminary directional control valve can be achieved.